IONIZATION 187 



the properties of a few important positive and negative ions, we 

 learn, at the same time, the properties in solution of all the elec- 

 trolytes of which these ions are constituents. This introduces a 

 great simplification into the study of chemistry. 



To obtain the necessary practice in dealing with double de- 

 compositions, the student should now return to the section on 

 these reactions in the preceding chapter (p. 173) and rewrite all 

 of the equations there given or referred to in full ionic form. 

 It is of great importance that the principles involved should be 

 thoroughly understood in each case. 



Ions and Conductivity. Pure water is an exceedingly poor 

 conductor of electricity. The high conducting power of a solu- 

 tion of an electrolyte is due, therefore, to the ions present. The 

 actual conductance of any given solution will depend on the 

 number of ions between the electrodes and the rate at which they 

 move. The more numerous the ions are, and the more rapidly 

 they migrate towards the oppositely-charged electrodes, the 

 greater will be the number of discharges taking place per second 

 upon each electrode. 



The rate at which the ions move, under given conditions, 

 has been carefully determined by methods which the student will 

 find discussed in text-books on physical chemistry. It will suffice 

 at this stage to give a few results. With electrodes one centi- 

 meter apart, and with a difference in electrical potential between 

 the positive and negative electrodes of one volt, the velocities 

 in centimeters per hour in dilute aqueous solution at 18 are as 

 follows: H+ 10.8, Na+ 1.26, Ag+ 1.66, OH~ 5.6, Cl- 2.12, NO 3 - 

 1.91. The hydrogen ion is the fastest, the hydroxyl ion holds 

 the second place. 



With the help of these figures we can calculate what is the 

 extent of ionization of the electrolyte in any solution containing 

 these ions, hydrogen chloride for example. We have already 

 learnt (p. 85) that a gram molecular weight of hydrogen chloride 



